There are two options for changing a redemption rate for a customer,First option would be giving that customer a special rate or the other or the next option would be

Select Number 2, just under the Search Bar, and search for your sale number that you want to print off a tax invoice Click the sale you are wanting, so

At the Home Screen you want to Select number Four, like the photo below, And Type in the stock number that you are wanting to change the price of and